5-Hydroxy-4-oxo-5-(2-oxopropyl)-4,5-dihydro-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylic acid (hereinafter, referred to also as “HOP”) is a compound represented by the following formula (2).

This substance is called an acetone adduct of pyrroloquinoline quinone in some cases. The structure of HOP is used for the determination of the structure of pyrroloquinoline quinone (for example, see Non Patent Literature 1). HOP is a white to yellow substance.